Day-night vision devices are used in particular for military purposes, where it is important for the devices to be convenient, fast and easy to operate and as compact and light as possible. While it was previously usual to change between day vision device and night vision device according to needs or to replace the standard day vision eyepiece by a “night vision attachment”, combined devices are preferably used today. This is advantageous in many respects; for example, carrying of the night vision attachment is superfluous and the change between day and night modes of the device is much faster.
For a device having both day and night vision function, in general a design comprising two separate channels—a day vision channel and a night vision channel—is usual. The day vision channel is generally formed in a known manner as a telescope comprising lens, optically refractive and reflective elements and eyepiece. The night vision channel is generally designed according to substantially the same principle and additionally has an image sensor—such as a light amplifier—and a high-speed lens. In the case of a low-light-level amplifier as an image sensor, the lens focuses the beams emanating from an object to be observed onto the entry window thereof. In the low-light-level amplifier, the image of the object is electronically amplified, therefore appears as a brighter visible image on the exit window and is projected by means of an eyepiece into the observer's eye.
Further functions are often desired for such a day-night vision device. Thus, for example, an image of a camera or location information, for example in the form of a map, can be input into the device via a display and superposed with the respective image in the device—projected by the day or night vision channel to the observer. A disadvantage of known devices is that this additional function has to be provided in duplicate, once for the day vision function in the day vision channel or via the day vision eyepiece and once for the device in its night vision form.